Benzene
CASRN 71-43-2 (endorsed 2013)
Guideline
Based on health considerations the concentration of benzene in drinking water should not exceed 0.001 mg/L.
General description
Benzene is a clear, colourless-to-yellow liquid and highly flammable aromatic hydrocarbon. It is present in petroleum products such as motor fuels and solvents, and motor vehicle emissions constitute the main source of benzene in the environment. Benzene occurs naturally in crude oil and coal and is an additive and a by-product of oil-refining processes. It constitutes approximately 1-2% of unleaded gasoline by volume (US DHHS, 2011). Tobacco smoke is another significant source of exposure (WHO, 2010). It also occurs in natural gas and emissions from volcanoes and forest fires.
Human exposure to benzene occurs primarily through inhalation (WHO, 2010). When released to surface waters, benzene rapidly volatilises to the air (WHO, 2010). Benzene is not persistent in surface water or soil and either volatilises to air or is degraded by bacteria under aerobic conditions (WHO, 2010). For water contamination, benzene is therefore of most concern in groundwater. Benzene can also occur in foods and drinks as a product of the reaction between benzoate and ascorbic acid, and has been found in soft drinks in the UK at concentrations as high as 0.028 mg/L (FSA, 2006).
Benzene is also used widely as an industrial solvent by the chemical and pharmaceutical industries in the production of styrene/ethylbenzene, cumene/phenol and cyclohexane. The use of benzene as a solvent has been greatly reduced in recent years.
Unlike other petroleum hydrocarbons such as ethylbenzene, toluene and xylene the odour threshold for benzene is relatively high at 10 mg/L (WHO, 2003).
Typical values in Australian drinking water
Benzene has only rarely been identified in Australian drinking waters. Natural concentrations in most water sources are usually very low. Benzene can occur naturally in groundwater as a result of proximity to, or contact with, coal seams, petroleum and gas deposits, and shales. It may be mobilised by extraction activities (Lesage et al., 1997; Leusch and Bartkow, 2011; Volk et al, 2011). However, contamination can occur, usually via exposure to petrochemicals in surface waters or groundwater. Known sources of groundwater and surface water contamination include leakage from sub-surface fuel storage tanks (do Rego & Netto, 2007) and proximity to natural hydrocarbon deposits (IPCS, 1993). Emissions of fuel components from boating use is a known source of contamination of multiple-use lakes and reservoirs (Schmidt et al., 2004). Benzene was reported in 9% of samples from an extensive groundwater survey undertaken in Denmark with the highest concentration being 0.034 mg/L (Juhler & Felding, 2003). Concentrations of up to 0.0027 mg/L were recorded in a NSW town water supply contaminated with petrol (Allen et al., 2005). Groundwater from a contaminated well in the USA contained up to 0.3 mg/L of benzene (IPCS, 1993). Benzene has been reported at up to 0.004 mg/L in municipal drinking water in Taiwan (Kuo et al., 1997), up to 0.01 mg/L in Germany (IPCS, 1993), and is occasionally detected in drinking waters in the USA (Williams et al., 2004).
Treatment of drinking water
Volatile organic chemicals such as benzene are most commonly treated in drinking water by aeration stripping and/or adsorption to granular activated carbon (GAC). A conventional biologically active sand filter has been shown to be highly effective for the removal of benzene from contaminated water, under suitable conditions (Arvin et al., 2004). Effective bioremediation of highly contaminated groundwaters has also been demonstrated (Sedran et al., 2004; Zein et al., 2006).
Measurement
A purge and trap gas chromatographic procedure can be used for the analysis of benzene (APHA, AWWA & WEF, 2012). An inert gas is bubbled through the sample and benzene is trapped on an adsorbent. The adsorbent is then heated and benzene analysed using gas chromatography with mass spectrometric (GC-MS) detection (Method 6200 B) or photoionisation (PI) detection (Method 6200 C) (APHA, AWWA & WEF, 2012). The method detection limit is 36 ng/L for GC-MS and 17 ng/L for GC-PI (APHA, AWWA & WEF, 2012).
Health considerations
Benzene is rapidly and efficiently absorbed (30-50%) following inhalation. Following ingestion, animal data indicate that nearly all is absorbed from the gastrointestinal tract. Less than 1% is absorbed through the skin. Following absorption it is widely distributed throughout the body. It is metabolised predominantly into phenol by the liver, and also by bone marrow (WHO, 2003).
Human health data are mainly from studies where benzene had been inhaled. Acute exposure to high concentrations affects the central nervous system causing dizziness, nausea, vomiting, headache and drowsiness. Inhalation of very high concentrations can cause death. Chronic and subchronic exposure to lower concentrations leads to a range of adverse effects on the blood system including pancytopenia, aplastic anaemia, thrombocytopenia, granulocytopenia and lymphocytopenia with white blood cells being the most sensitive (WHO, 2003; Health Canada, 2009). There is considerable evidence that occupational exposure to low benzene concentrations in air for periods as short as 1-5 years may result in leukaemia (ATSDR, 2007).
In animal studies, benzene caused leukaemia and other cancers when administered orally and by inhalation to rats and mice. It can also induce chromosome damage and gene mutation in mammalian cells. It was not found to be mutagenic in tests with bacteria.
The International Agency for Research on Cancer has concluded that benzene is carcinogenic to humans (Group 1, sufficient evidence of carcinogenicity in humans) (IARC, 1987).
Derivation of guideline
The European Union (1998), WHO (2011), Health Canada (2006), USEPA (2008) and New Zealand (MoH NZ 2008) have set drinking water guidelines for benzene of 0.001-0.01 mg/L based on carcinogenic potential (leukaemia) of benzene in humans from inhalation associated with occupational exposures and/or a 2 year oral study in rats and mice (NTP 1986).
USEPA (2003) derived a cancer slope factor (CSF) of 0.015 to 0.055 per mg/kg/day based on linear extrapolation of leukaemia data from occupational exposure. This translated to a lifetime risk of one excess cancer case per 1 million people associated with a concentration of 0.001-0.01 mg/L of benzene (USEPA 2003).
Both USEPA (2003) and Health Canada (2006) identified that inhalation of volatilized benzene and dermal adsorption need to be added to ingestion of drinking water. Based on exposure from showering and bathing, Health Canada (2006) derived equivalent doses of 1.2L water per day to account for inhalation and 0.8L water per day to account from dermal adsorption. Using these equivalent doses and the USEPA (2003) cancer slope factor a guideline value can be calculated using the formula:
where:
0.015 -0.055 mg/kg/day is the CSF range calculated by USEPA (2003) from occupational exposure to benzene
70 kg is the average weight of an adult
is the additional lifetime risk of one cancer from drinking water exposure
4L/day is the average dose including 2L/day for ingestion plus 1.2L equivalent dose/day for inhalation and 0.8L equivalent dose/day for dermal adsorption.
WHO (2003) also used the occupational leukaemia data to determine that a concentration of 0.001 mg/L in drinking water would entail a maximum lifetime risk of one additional case of cancer per 1 million people. Analysis of data from a 2 year gavage study in rats and mice (NTP 1986) using the robust linear extrapolation model produced similar results with an excess lifetime cancer risk of 1 per million people associated with 0.001-0.008 mg/L benzene based on leukaemia and lymphomas in female mice and oral cavity squamous cell carcinomas in male rats respectively (WHO, 2003). On the basis of these two calculations WHO (2003) identified that concentrations of 0.01 mg/L and 0.001 mg/L were associated with excess cancer rates of 1 per 100,000 and 1 per 1,000,000 people respectively. WHO (2011) adopted a guideline value of 0.01 mg/L based on an estimated additional lifetime risk of one cancer per 100,000 people.
The concentration of 0.001 mg/L associated with an excess cancer risk of 1 per 1,000,000 people calculated by WHO (2003) is within the range of 0.00032-0.0012 mg/L. For consistency with WHO (2003, 2011), a health-based guideline of 0.001 mg/L has been adopted.
References
Agency for Toxic Substances and Disease Registry (ATSDR) (2007) Toxicological profile for Benzene. Public Health Service, US, Department of Health and Human Services, Atlanta, Georgia.
Allen, K., Thorne, S. and Byleveld, P. (2005) You found benzene - where? Benzene in source drinking water in northern New South Wales. Presented at Australian Water Association Specialty Conference “Contaminants of Concern in Water”, Canberra 22-23 June, Australia Water Association, Canberra.
American Public Health Association (APHA), American Water Works Association (AWWA) and Water Environment Federation (WEF) (2012). Standard Methods for the Examination of Water and Wastewater, 22nd Edition. Eds. Rice EW, Baird RB, Eaton AD and Clesceri LS.
Arvin, E., Engelsen, P. and Sebber, U. (2004) Biodegradation of gasoline compounds (BTEX) in a water works sand filter. Water Science & Technology: Water Supply, 4(5-6), 29-33.
do Rego, E. C. P. and Netto, A. D. P. (2007) PAHs and BTEX in groundwater of gasoline stations from Rio de Janeiro City, Brazil. Bull. Environ. Contam. Tox., 79(6), 660-664.
European Union (1998) Council Directive 98/83/EC on the quality of water intended for human consumption. Official Journal of the European Communities. L330.
FSA (2006). Survey of benzene in soft drinks. Food Survey Information Sheet No 06/06 March 2006. Food Standards Agency, UK
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Juhler, R. K. and Felding, G. (2003) Monitoring methyl tertiary butyl ether (MTBE) and other organic micropollutants in groundwater: Results from the Danish National Monitoring Program. Water Air Soil Poll., 149(1-4), 145-161.
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MoH NZ (2008) New Zealand Ministry of Health, Drinking-water standards for New Zealand 2005 (Revised 2008), p 9. Ministry of Health, Wellington.
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Sedran, M. A., Pruden, A., Wilson, G. J., Suidan, M. T. and Venosa, A. D. (2004) Biodegradation of methyl tert-butyl ether and BTEX at varying hydraulic retention times. Water Environ. Res., 76(1), 47-55.
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